Several types of converters are known for use in power supply systems, where there is a need to convert an AC power to a controlled DC power. The AC power will usually be supplied from an AC power source, such as the mains. The DC power is supplied to equipment such as telecommunication equipment, broad band data communication equipment (GSM/UMTS base stations etc), military equipment, medical equipment etc.
There are several requirements for such power supply systems. First of all, the efficiency should be high, i.e. the losses should be low. The power supply system described in WO 2009/028954 and WO 2009/058024 has an efficiency of ca 96% and is marketed and sold by Eltek Valere under the name FlatPack 2 HE. The power supply system provided as a unit for insertion into a rack. The unit has an height of 1 U (the standard height of one shelf in a rack, corresponding to 44.5 mm), a length of 328 mm and a width of 109 mm so that four such units may be provided next to each other in a 19″ rack. The unit may deliver a power of 2 kW or 3 kW at −48V DC.
One object of the next generation power supply is to provide a smaller unit having substantially the same power as the above power supply system and with a relatively high efficiency. More specifically, the new unit should be 1 U high. The length should be 220 mm so that the unit and the rack may be provided in a 30 cm power cabinet. In addition, the width should 72 mm in order to provide that six such units may be provided next to each other in a 19″ rack. The unit should be able to supply a power of 2-3000 W at −48V DC. Hence, the available volume for the components is reduced by approximately 55%.
Another object of the next generation power supply system is to reduce costs. One contribution to cost reduction is the reduced size. Another contribution to cost reduction is to use cheaper electronic components such as processor units etc.
However, such electronic components are simpler, and consequently, effort must be put into using the electronic components smarter. One way of achieving this is to reduce the number of calculations needed for controlling the power supply system.
The power supply system comprises a fan for blowing air through the unit. The fan is normally located high the front side of the unit and blows air out through the rear side of the unit. The increased power density (power per volume unit) of the unit makes it difficult to achieve a satisfying air flow through the unit.
In order to be able to achieve the objects above, a redesign of the printed circuit card and the organization of the electronic components were necessary. In FIG. 1, the prior art DC-DC converter 1 shown in WO 2009/028954 is disclosed, with a LLC resonant circuit indicated by a dashed box 2 and a synchronous rectifier indicated by dashed box 3. The LLC resonant circuit 2 comprises two switches Sdc1, Sdc2 having respective gates Gsdc1, Gsdc2, and the synchronous rectifier 3 comprises two switches Sr1, Sr2 having respective gates Gsr1, Gsr2. These switches are controlled by a digital signal processor or DSP connected via a gate driver. Hence, gate control terminals GCsr1, GCsr2, GCdc1, GCdc2 of the DSP controls the state of respective switches.
To keep the costs low, it is desired to use low end digital signal processors to control the switches. One example of such a low end DSP is the Texas Instruments TMS320F28032PAGT, having a cost of less than $5 per unit. However, in order to be able to use such DSPs, they must be programmed smarter. Still, it has turned out that such DSPs is not capable of handing failure situations in a satisfying way, due to a relatively low number of instructions per second in the DSP.
During normal operation, the output current Iout is positive. However, if a failure situation occurs, for example due to a short circuit of the load, this current will be negative. In such situations, the switches Sr1 and Sr2 of the LLC resonant circuit must be turned low quickly in order to protect the components of the converter. However, forcing the switches Sr1 and Sr2 low may result in bad field performance.
One or more embodiments of the invention provide a DC-DC converter which solves the above problems and which makes it possible to provide a reliable power supply unit with the specifications mentioned above.